This invention relates to the static compensation of reactive power in a three phase system in using converters in general, and more particularly to an improved method and apparatus for carrying out such static compensation using control variables for the reactive power converters which are derived from load currents.
With the recent availability of reactive power converters, the common manner of obtaining reactive power compensation using rotating machines has been replaced to a large extent. Typically, in a system where reactive power compensation is required, a capacitor battery, for the three phases, and a three phase set of converters having short circuited, controlled reactive converters are employed. It is well known that short circuited converter constitutes a reactive power load with the reactive power consumption depending on the control angle of the converter rectifiers. In an installation such as this with capacitor batteries and converters the capacitor battery is designed so that it can compensate for the maximum reactive power. The short time fluctuations of reactive power are compensated for by the reactive power converter.
An application in which it is particularly difficult to compensate for reactive power is in electric furnaces. Such furnaces due to their design and operational behavior cause considerable fluctuations of active and reactive power. In addition, these power fluctuations are not distributed symetrically over the individual phases of the three phase system supplying the furnace.
Various means for operating a reactive power compensating device are known. In the method disclosed in Siemens Zeitschrift, 1973, pages 706 to 711 the control variables for the reactive power converter are derived from the measured reactive power. Reactive power is controlled by means of two series connected control loops, namely, an inner current control loop and an outer reactive power control loop which supplies the control variables for the converter. This control concept requires an expensive means for determining reactive power using multiplication of the line current by a voltage which is orthogonal to the respective phase voltage. In addition, smoothing of the determined value of reactive power must be provided. This smoothing results in time constants which are troublesome particularly in compensating occurring reactive power pulses. Separate control for the individual phases of the three phase system supplying the load is not possible with this known method.
In the article "The Generation of Reactive Power by Synchronous Machines" by G. Hosemann VDE-Verlag Berlin, 1963, pages 121 and 122, an arrangement for connecting an arc furnace, which is an asymmetrical load, together with a set of balancing reactive power machines to the three phase system is disclosed. In order to compensate for the counter-rotating current component, the rotor of one reactive power machine is driven in the counter-rotating sense and is equipped with two mutually orthogonal windings. Each of the windings is excited by means of a converter whose control variables are the components of the counter-rotating load current. The voltage fluctuations caused by the co-rotating component of the load are compensated by a driving synchronous motor used as an additional reactive power machine. The co-rotating and the counter-rotating current components are determined using a component bridge. This previously developed reactive power compensation system requires an expensive set of machines, i.e., requires two reactive power machines. The compensation of reactive power fluctuations still occurs relatively slowly when the reactive power machines are excited by means of static converters.
In view of these various difficulties the need for an improved static reactive power compensation system which permits balancing the reactive power in each of individual phases of the supply system and which permits rapid compensation of changes of reactive power which occur suddenly is evident.